The sun produces radiation in a variety of wavelengths, many of which are not visible to the human eye. FIG. 1 shows a solar radiation spectrum 100. Sun 105 outputs radiation (or light) in a variety of wavelengths, including infrared light 130, visible light 125, ultraviolet A light 120, ultraviolet B light 115, and Ultraviolet C light 110. Ultraviolet A (UVA) light has wavelengths between 315 to 400 nanometers. Ultraviolet B (UVB) light has wavelengths between 280 to 315 nanometers. Ultraviolet C (UVC) light has wavelengths between 100 to 280 nanometers. UVA, UVB, and UVC light are invisible to the human eye. Visible light has wavelengths between 400 to 780 nanometers. Infrared has wavelengths between 780 nanometers and 1 millimeter and is invisible to the human eye.
FIG. 2 shows the transmission and absorption of component spectra in radiation spectrum 100 as the radiation travels from the Sun through the Earth's atmosphere and encounters human skin. Human skin 240 is composed of two layers—the epidermis 220 (i.e. outer lay) and the dermis 235. UVA and UVB light interact with human skin in different ways and cause different maladies. Both UVA and UVB light can harm a person's skin. UVA light, which can penetrate human skin more deeply than UVB light, is known to play a role in skin aging, wrinkling, and skin cancer. UVB light is the chief cause of skin reddening and sunburn, and also plays a key role in the development of skin cancer. Among other factors, the skin's response to UVA light and UVB light depends to some degree upon skin type, e.g., whether a person's skin is lighter or darker. Darker skin has more melanin, which offers a higher degree of protection from UV light.
Referring to FIG. 2, UVC light 110 can be transmitted 205 and absorbed by the Earth's atmosphere 210. Some UVB light 115 also can be transmitted 215 and absorbed by the Earth's atmosphere 210, but a portion is transmitted 215. UVA light 120 generally is not absorbed by the Earth's atmosphere 210 and is transmitted 230. UVB 215 can be absorbed by epidermis 220. UVA 120 can penetrate the epidermis 220 into the dermis 235.
Sunscreens (e.g. sunblock) are topically-applied lotions made to protect a person's skin from exposure to UVA light and/or UVB light. Sunscreens may include chemical or physical filters. Chemical filters form a thin, protective film on the skin's surface to absorb the UV radiation before it penetrates the skin. Physical filters are particles that are disposed on the skin's surface and reflect UV light.
Even broad-spectrum sunscreens that protect against UVA light and UVB light provide only limited protection. FIG. 3 illustrates coverage problems associated with improper sunscreen application. Picture 300 shows a boy 315 playing at the beach. The boy 315's current UV coverage is inconsistent. For example, spots 310, 320, 325, and 330 correspond to locations in which the sunscreen coverage is less effective. Such spots can result from a variety of factors, including improper application, dissipation from activity or swimming, and uneven wear. As a result of coverage problems, undetectable imbalances in protection frequently arise.